Install FreeBSD sources.

In some cases you may need the sources (kernel source) for your FreeBSD
installation. However if you did not select the source package in the
installer it will not be installed.

The installation is actually quite simple but is not done via a tool as some
might expect. In former times you could use sysinstall but that does not
exist anymore.

You can either connect via your favourite ftp client (lftp) to the
official server (or a mirror of your choice) or you can use the fetch
command to retrieve the source package. Be sure to download the correct
version (e.g. use freebsd-version -k to find out).